Scientists have uncovered yet more evidence that the appendix, that odd wormlike attachment to the large intestine, may not be a useless organ after all.
Instead, the new evidence strengthens the idea that the appendix may be a storehouse for beneficial microbes that help fight off infections in the gut.
Charles Darwin helped popularize the idea that the appendix was a vestigial structure. During his lifetime, only humans and other great apes were known to have an appendix. This led Darwin to hypothesize that the appendix was an evolutionary artifact — a remnant of the days when our ancient ancestors needed a larger cecum (the pouch-like beginning of the large intestine) to store bacteria to break down plant tissues. Darwin believed that when those ancestors switched to a more easily digestible fruit-based diet, the cecum shrank — and the appendix became unnecessary.
In a new study, published online this month in Comptes Rendus Palevol, the researchers compiled information on the diets of 361 living mammals, including 50 species now considered to have an appendix, and plotted the data on a mammalian evolutionary tree. They found that the 50 species are scattered so widely across the tree that the structure must have evolved independently at least 32 times, and perhaps as many as 38 times.
By plotting the dietary information onto the evolutionary tree, the researchers could work out whether the appendix appears when a particular group of mammals changes its diet, suggesting appendix evolution doesn’t necessarily proceed as Darwin thought. In most cases, there was no sign of a dietary shift. He may have correctly identified the origin of the ape appendix, though, which the analysis confirms did appear when our ancestors switched diets.
As Barres notes, “the hunt is now on” to identify exactly what the function of the appendix is:
The research team [which includes Dr. William Parker, a surgeon who studies the immune system at Duke University] may already have the answer. In 2007, Parker and his colleagues suggested that the appendix has an immunological role, acting as a “safe house” for beneficial gut bacteria. These bacteria help train the immune system and can prevent diseases by outcompeting dangerous pathogenic bacteria — but there are times when the dangerous microbes gain the upper hand and overrun the gut. The researchers reasoned that when this happens, the beneficial bacteria could retreat to the safety of the appendix, which remains unaffected. Once the immune system has beaten the infection, the beneficial bacteria emerge from the appendix to quickly recolonize the gut.
As I noted in Second Opinion last year, another study by Parker found that among people who had developed a Closteridium difficile infection, those without an appendix were four times more likely to have a recurrence of the bacterial infection than those who still had their appendix. C. difficile kills an estimated 14,000 Americans each year. Most are older adults who become infected while being cared for in a hospital, nursing home or other medical setting.
The Comptes Rendus Palevol study is behind a paywall, but you can read Barres’ account of it at the ScienceNow website.